Does anyone know how to install the Drone Deploy application to the built in phone on the Phantom 4 Pro +

No.
First you have to develop a custom ROM for the Rockchip SOC.

Hello. DJI does not support 3rd party application. We always recommend to use application that is tested and recommended by DJI. We also have our GS Pro (Ground Station Pro) that has the same feature with the Drone Deploy. However, this app does not support the Phantom 4 Pro +, since it is not compatible with Android OS. It only works with iOS devices. Sorry for the inconvenience.

Is there any forecast when a p4p mapping app will be released? It is somewhat absurd not to have a mapping feature on the more expensive version of phantom 4.
It seems to me that DJI really does not mind giving new uses to the drones of their customers, unfortunately. People who have invested in p4p should be content to take photos and videos. DJI punished us for investing in the best product in the phantom line.

Is there any forecast when a p4p+ mapping app will be released? It is somewhat absurd not to have a mapping feature on the more expensive version of phantom 4.
It seems to me that DJI really does not mind giving new uses to the drones of their customers, unfortunately. People who have invested in p4p should be content to take photos and videos. DJI punished us for investing in the best product in the phantom line.

cadfjr Posted at 4-30 05:43
Is there any forecast when a p4p+ mapping app will be released? It is somewhat absurd not to have a mapping feature on the more expensive version of phantom 4.
It seems to me that DJI really does not mind giving new uses to the drones of their customers, unfortunately. People who have invested in p4p should be content to take photos and videos. DJI punished us for investing in the best product in the phantom line.

You've just assumed that the + model is "better" because it cost more.
But it only costs more because it has the screen included.
Buy a non + version and a screen and you can easily pay more.

There is no forecast for anything.
The + model has been around for 2.5 yrs now and if it was ever going to be changed to accept 3rd party apps, it would have happened long ago.
Accept that it isn't going to happen.
Buy a regular controller and tablet and use that if you need to run a mapping app.